Seat belt problem

Hi, First visit and hoping someone can help please.

I have a 2000 (x reg) Clio 1.2 5 door. The offside rear seatbelt has jammed - can't pull it out and can't retract it. I have tried repeatedly pulling and releasing it to try to free it but without success. I have unscrewed the plastic cover through which it feeds but cannot remove it: seems to be held by 2 large plastic pegs, only 1 of which will come out easily, and the edge is trapped under the door trim. I do not want to risk breaking it, but can't get to the seat belt mechanism to have a go at freeing it.

Any advice would be much appreciated please - I'm due to take teenage daughter and friends to their first gig in two days, and can't afford to take it to a garage after paying for her ticket!

Thanks in advance, pdpnuts